The joints of a human skeletal structure typically comprise a ball and a socket. Joints, such as the hip and shoulder, provide for universal motion, i.e., relative motion about three transverse pivot axes.
For example, a hip joint includes a femoral head which is joined to the proximal femur by a neck which is angularly disposed relative to the axis of the femur and relative to the vertical axis of the human body. A natural socket or acetabulum receives the femoral head and cooperates therewith to form a universal joint which permits relative motion about three transverse pivot axes.
Various progressive diseases, such as osteoarthritis can bring about deterioration of the natural socket and/or the natural femoral head. When this occurs, the diseased component can be replaced or rebuilt using an appropriate prosthetic device. For example, in total surface replacement, an acetabular cup is cemented into the natural socket or acetabulum, and a femoral cup is cemented over the natural femoral head after reaming. The femoral cup fits into the acetabular cup to provide the necessary universal motion.
In total hip replacement, an acetabular cup is cemented into the acetabulum, and the natural femoral head and neck are removed. A femoral member comprising an elongated stem, a neck and a head is mounted within the proximal femur by cementing of the stem into the femur. The head is received in the cup to provide the desired universal motion. One example of this construction is shown in Chambers U.S. Pat. No. 3,656,184.
In another form of total hip replacement, a femoral cup is mounted on the head of the prosthetic femoral member. The femoral cup is received within the acetabulum. Constructions of this type are shown by way of example in the following U.S. patents:
U.S. Pat. No. 3,813,699 to Giliberty PA1 U.S. Pat. No. 4,172,296 to D'Errico PA1 U.S. Pat. No. 4,214,463 to Khovaylo
In each of these constructions, the head is mounted within the femoral cup for universal movement, and similarly, the femoral cup is mounted for universal movement within the acetabulum. Accordingly, as the hip joint is used, the motion may be accommodated by either or both of these interfaces to the extent allowed by the ratio of the head-to-neck diameters and the overlap of the femoral cup past the equator of the femoral head.
Unfortunately, in use of this latter system, a pseudocapsule may form and impede the movement of the femoral cup in the acetabulum. In this event, motion occurs primarily or entirely between the head and the femoral cup, and this can accelerate deterioration of the cartilage by uneven lubrication between the femoral cup and the natural acetabulum and the creation of local stresses in the natural acetabulum due to the lack of relative movement between the femoral cup and the natural acetabulum. Thus, leaving to chance the interface which accommodates the hip motion may accelerate the deterioration of the joint.
The stem of the femoral member may include flanges or fins extending along the lateral, i.e., outer proximal edge of the stem. The fins increase the strength of the femoral member and enhance bonding of the stem within the intermedullary canal of the femur. For example, fins of this type are shown in Charnley U.S. Pat. No. 4,021,865. As disease progressively worsens, it may become necessary to replace the femoral member. Unfortunately, the fins of the prior art make the femoral member difficult to remove from the femur due to the location nd curvature of the fins.